1. Field of the Invention
The present invention relates to a novel tetra-block vinyl aromatic hydrocarbon (S)-conjugated diene (B) copolymer and to compositions containing such tetra-block copolymer. More particularly, the present invention relates to a tetra-block styrene (“S1”)-butadiene (“B1”) copolymer having B1-S1-B2-S2 block copolymer configuration and to compositions containing such tetra-block styrene-butadiene copolymer. The present invention also relates to a novel method of making compositions containing the B-S-B-S block copolymer in a single polymerization step.
2. Technical Background
Preparation of linear block copolymers of vinyl aromatic hydrocarbons and conjugated dienes, such as styrene and butadiene, is known. One of the first developments were linear S-B-S block copolymers made with styrene and butadiene. Several other variations for block copolymer structures and methods of preparation have been found since then.
A number of thermoplastic elastomers such as styrene-butadiene block copolymers, are produced by a multi-step process which includes anionic polymerization. Styrene-butadiene block copolymers may have either a star or a linear configuration. There are generally three different types of linear copolymers produced by anionic polymerization: (1) tapered block; (2) di/tri-block; and (3) random.
Tapered or graded block styrene-butadiene copolymers are typically formed when alkyllithium catalysts, styrene and butadiene are mixed in a batch reactor. Random styrene-butadiene copolymers are typically formed when the anionic polymerization is carried out in a continuous flow reactor.
Thermoplastic elastomers (TPE), such as di-block or tri-block styrene-butadiene copolymers, are typically formed when polymerization is carried out in a semi-batch reactor by sequential addition of monomers. Polymerization of TPE may also be carried out through non-sequential addition of component monomers. Because of the stability of the “living” nature of the allylic lithium end group, butadiene-styrene copolymers of widely different structures and properties can be prepared.
For example, in styrene-butadiene-styrene (S-B-S) tri-block copolymers, the rubbery soft B block is between the two hard S blocks. The arrangement of hard and soft blocks yields commercially useful properties. These copolymers have two phases, two glass transition temperatures and are characterized by high raw green strength, complete solubility and reversible thermoplasticity. S-B-S tri-block copolymers are typically produced by a multi-step process which includes first polymerizing styrene to form the S block followed by polymerizing half of the butadiene to form the half B block. Then a di-functional coupling agent is added to link the living polymer chains and form the tri-block polymer.
One method of producing a S-B-S tri-block copolymer is as follows:                (a) charge the batch reactor with styrene and solvent;        (b) add the n-butyl lithium (BuLi) initiator and allow the styrene to polymerize;        (c) add butadiene and allow the butadiene to polymerize;        (d) add a di-functional coupling agent and allow time for coupling; and        (e) add a terminator to terminate the remaining initiator and live polymer chains.        
Prior techniques have taught the process of mixing a diene monomer, such as butadiene, and a mono-vinyl aromatic hydrocarbon monomer with a lithium initiator compound to catalyze the polymerization reaction. For example, one butadiene polymerization process includes the steps of: (1) mixing 1,3 butadiene with lithium amine initiator compounds, optionally in the presence of a modifier, wherein the lithium amine initiator compound is selected from the group consisting of compounds of the formulas R′—N—Li wherein R′ is a cyclic amine group having 3 to 18 ring carbon atoms and, together with the nitrogen, a ring of 4 to 19 atoms; (2) effecting polymerization conditions; (3) terminating polymerization with a terminating agent to form a functionalized diene elastomer; (4) compounding the functionalized diene elastomer with an amorphous silica filler, a carbon black filler, or both, and a vulcanization agent; and, (5) effecting vulcanization of the filler, functionalized diene elastomeric compound. Useful polymerization initiators include mixtures of metal amides such as lithium amides, and more particularly including litho-hexamethyleneimine (LiHMI).
Such polymerization may be conducted in an acyclic alkane solvent, such as the various hexanes, heptanes, octanes, mixtures thereof, and the like. Where desired, a modifier, such as a polar modifier, may be added to the polymerization ingredients in order to promote randomization in copolymerization and to control vinyl content. Suitable amounts of modifier range between 0 and 90 or more equivalents per equivalent of lithium. The amount depends on the amount of vinyl desired, the level of styrene employed and the temperature of the polymerization, as well as the nature of the specific polar modifier employed.
The modifier compounds which are commonly used in combination with the anionic polymerization initiators such as lithium amides, are represented by the modifier compounds structural formulas I and II shown in FIGS. 1 and 2 respectively, wherein R1 and R2 independently are hydrogen or an alkyl group and the total number of carbon atoms in —CR1R2—is between one and nine inclusive; y is an integer of 1 to 5 inclusive, y′ is an integer of 3 to 5 inclusive, R3′, R3, R4 and R5 independently are —H or —CnRH2n+1 wherein n=1 to 6. While the modifiers of structural formula I are linear oligomers and the modifiers represented by structural formula II are cyclic oligomers, hereinafter the term oxolanyl modifiers is contemplated to encompass the modifiers of both structural formulas.
Suitable oxolanyl modifiers for use in combination with lithium amide initiators include: bis(2-oxolanyl) methane; 2,2-bis(2-oxolanyl) propane; 1,1-bis(2-oxolanyl) ethane; 2,2-bis(2-oxolanyl) butane; 2,2-bis(5-methyl-2-oxolanyl) propane; 2,2-bis-(3,4,5-trimethyl-2-oxolanyl) propane. These modifier compounds represent a few of the dimer compounds represented by structural formula I and other linear and cyclic oligomer modifiers are apparent from their structural formulas. Other useful modifiers include tetrahydrofuran (THF), dialkyl ethers of mono and oligo alkylene glycols; “crown” ethers; tertiary amines such as tetramethylethylene diamine (TMEDA); linear THF oligomers and the like.
Thermoplastic elastomers made of multi-block styrene-butadiene copolymers have been also developed using lithium amide initiators and suitable modifiers to catalyze styrene and butadiene monomers. Typically the process to manufacture multi-block styrene-butadiene copolymers is a multi-step process. One solution polymerization process for making the multi-block copolymers includes the sequential polymerization of styrene monomers and then of butadiene monomers onto the polymerized styrene blocks. A suitable solvent for the polymerization is cyclohexane. This results in the polymerization of first the styrene and then of the butadiene. In a second embodiment, the styrene monomers and the butadiene monomers are polymerized separately and then the polymer blocks of styrene are coupled to the polymer blocks of butadiene.
An issue with known styrene-butadiene block copolymers is that the copolymer includes an undesirable by-product having the configuration of a thermally coupled B1-S1-B2 block copolymer. This by-product makes the above-block copolymers waxy and reduces the strength of the material.
It is known that polymers can be used to modify the rheological properties of asphalt. Asphalt generally includes asphalt materials, frequently referred to as “bitumen (binder).” Asphalt concretes refers to compositions of aggregates and/or filler materials combined with asphalt. Herein the term “asphalt” shall generally refer to both bitumen and to asphalt concretes. Asphalt compositions includes compositions of asphalt and asphalt modifiers, including polymeric asphalt modifiers.
In general, polymeric asphalt modifiers may be viewed as dispersed systems that create a polymer network but that remain in phase from the original asphalt cement or may be viewed as reacted systems characterized by a chemical reaction between the polymer and the asphalt. STEREON® 210 (available from Firestone Polymers of Akron, Ohio) and Dynasol 1205 (available from Dynasol, of Altamira Tamaulipas, Mexico) are examples of common styrene/conjugated-diene block polymer compositions used commercially as polymeric asphalt modifiers.